The characterization of a number of yeast mutants has been continued. Mutants resistant to trichodermin have been shown to have an altered protein of the 60S subunit. Several other antibiotics are being studied in terms of their mode of action in the yeast and the wheat germ cell-free systems. S. cerevisiae mutants resistant to blasticidin S, MDMP an hygromycin B have been isolated and are being studied. Several cold sensitive mutants of yeast have been shown to produce ribonucleoprotein particles at the non-permissive temperature. The nature of these particles and their relationship to the biosynthesis of complete ribosomes is unknown. Protein kinase mediated modification of yeast ribosomes has been shown to occur to a substantial extent both in vitro and in vivo. Phosphorylation of 80S particles and isolated 40S and 60S subunits indicates that extensive unfolding of the ribosomes can occur on subunit formation.